The use of thermally conductive plates with embedded temperature sensing devices is well known as an apparatus for detecting misalignment of any moving belt such as a conveyor belt and transmitting an electrical signal or other indication to a remote location, to allow the misalignment to be corrected or the conveyor shut down before the heat buildup of the conveyor belt or parts of the machine against which the misaligned belt may rub reaches a dangerous level. These detection devices are often referred to as rub blocks. The action of the belt rubbing against the rub blocks also causes wear of their thermally conductive plates. If such wear goes undetected, the belt can rub completely through one or both plates, rendering the apparatus inoperative or it could cause the temperature sensing element of the rub blocks to fail, again rendering the apparatus inoperative, and further or continued misalignment of the conveyor belt could go undetected.
To ensure that the detection apparatus is still working correctly, it is necessary for frequent visual inspections to be made to ensure that the apparatus has not been excessively worn. In environments where the apparatus is mounted on an enclosed conveyor or belt or bucket elevator or similar machines, it is not possible to make a visual inspection of the apparatus without removing it from the machine casing. Alternatively, the apparatus has to be mounted on a hinged door so that it may be accessed for inspection. No matter how frequent such inspections may be, they are not continuous and therefore it is likely that an undesirable condition will be undetected for some time.
These inspections would normally necessitate frequent machine shutdown for safety reasons and are costly and frequently omitted. If there is enough space available on the machine, it is possible to use more than one rub block apparatus mounted adjacently, with one apparatus protruding into the machine casing more than the second apparatus. In this way, the theory is that the apparatus which protrudes the most would become worn first and subsequent rubbing on the second apparatus would indicate that the first apparatus has worn excessively and needs replacement. However this arrangement is not normal practice due to the additional cost of installation of the additional apparatus, the cost of the additional apparatus, the additional space needed for the additional apparatus, and the fact that wear at the location of the additional apparatus is not the same wear at the location of the initial apparatus even when the apparatuses are placed within close proximity to each other.
This apparatus of the present invention provides for two levels of rub detection within a single apparatus such that only one device need be fitted. The invention is able to provide two signals or one combined signal of both rub conditions and wear conditions. The signals can be processed to provide either two levels of alarm or one level of alarm followed by an automatic stopping signal for the machine. This invention therefore provides increased effectivity and cost savings by eliminating extraneous cost of a required second adjacent apparatus and without the cost of frequent inspections or hinged doors for inspection.